When you go into a yarn shop or crafts store, it's easy to be overwhelmed by all the choices. Which yarn is right for your project? Most people stick to the exact type (and even the color!) of yarn suggested in the pattern, but that's not much fun.
It's also not necessary as long as you understand something about yarn weights.
The Standard Yarn Weight System
Yarn weight refers to the thickness of the yarn.
It's a range, from super fine to super bulky. There are six different categories of yarn weights, and according to the Craft Yarn Council, specific weights of yarn should produce a somewhat predictable number of stitches when using a particular sized needle.
The higher the number, the heavier the yarn and the fewer stitches per inch you will get.
Further reading: Does Ply Have Anything to Do with Yarn Weight?
Why Standards Matter
This is where knitting gets fun. If you know that every bulky yarn is going to give you around the same number of stitches (in this case, 12 to 15 stitches in four inches on size 9 to 11 needles) and you have a pattern that uses bulky yarn and size 10 needles, you can buy any kind of bulky yarn and get a similar result.
The difference between 12 stitches per four inches and 15 stitches is still pretty huge when you're trying to make a sweater fit.
Determining Yarn Weight
Most yarn manufacturers make it easy for you to determine the weight of a particular yarn. Many of the mass-produced yarns use the yarn standards ranking system and will have the number and weight printed right on the label.
Other manufacturers don't make it as easy, but they should have a gauge statement that will say something like "24 stitches and 22 rows per four inches on size four needles." If you know a little bit about yarn weights (which you will when you consult the chart below) you'll know that the yarn in question is sport weight.
|Yarn Weight||1: Super Fine||2: Fine||3: Light||4: Medium||5: Bulky||6: Super Bulky||7: Jumbo|
|Yarns in Category||Baby, Fingering, Sock||Baby, Sport||DK, Light, Worsted||Afghan, Aran, Worsted||Chunky, Craft, Rug||Bulky, Roving||Jumbo, Roving|
|Needle Size (US)||1-3||3-5||5-7||7-9||9-11||11-17||17 and up (or arms)|
|Average stitches per 4 inches||27-32||23-26||21-24||16-20||12-15||7-11||6 and fewer|
Source: Yarn Standards